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The influence of mean stress and mean strain on fatigue damage following overloads of yield stress magnitude was investigated. Strain controlled loading spectra each consisting of a periodic fully-reversed overload cycle followed by a number of constant amplitude small cycles were applied to normalized SAE 1045 steel and 2025-T351 aluminum axial specimens. The small cycle strain range was kept constant for all tests but their position within the overload hysteresis loop was varied to observe the effect of mean stress on fatigue damage. The crack opening stress and strain were calculated as a function of the number of cycles after the overload and the effective stress range was defined as the difference between the maximum stress and the greater of either the minimum stress or the crack opening stress. Fatigue life estimates were made using a linear cumulative damage summation based on the effective stress-life fatigue curves for these materials.

This paper presents a model for fatigue life prediction for metals subjected to variable amplitude service loading. The model, which is based on crack growth and crack closure mechanisms for short fatigue cracks, incorporates a strain-based damage parameter, EΔε*, determined from the effective or open part of a strain cycle along with a fatigue resistance curve that takes the form: EΔε* = A(Nf)b, where E is the elastic modulus, Nf is the number of cycles to failure, and A and b are experimentally determined material constants. The fatigue resistance curve is generated for a SAE 1045 steel and the model is used successfully to predict the fatigue lives of smooth axial specimens subjected to two variable amplitude strain histories. The model is also used to predict the magnitude of non-damaging cycles that can be omitted from the strain histories to accelerate fatigue testing.